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Für den sicheren Betrieb eines Brennstoffzellenfahrzeugs sind Sensoren wichtig, welche die Wasserstoffkonzentration im Fahrzeuginterieur kontinuierlich überwachen, um möglichen Entzündungen bzw. Explosionen vorzubeugen. Dafür wurden von verschiedenen nationalen und internationalen Instanzen bestimmte Kriterien definiert, die Wasserstoffsensoren für einen Einsatz in Brennstoffzellenfahrzeugen erfüllen müssen. Im Jahr 2020gab es noch keinen einzigen kommerziell erhältlichen Sensor, der alle diese Kriterien bedienen konnte.
In der vorliegenden Arbeit wurde für zwei verschiedene Sensoren die Eignung des thermischen Wirkprinzips für die Detektion von Wasserstoff im Kontext dieser Sicherheitskriterien untersucht. Dabei wurden mit einem MEMS-Sensor und einem Mikrodrahtsensor zwei vergleichsweise einfache Geometrien herangezogen, welche den eindimensionalen und zylindersymmetrischen Fall abbilden sollen. Für zwei dynamische Operationsmodi (gepulste Ströme und 3ω-Methode) wurden darauf aufbauend analytische Modelle basierend auf vereinfachten Sensorgeometrien entwickelt, um die Temperaturentwicklung des
Sensorelements in Abhängigkeit der Zeit zu charakterisieren. Diese Studien wurden anschließend anhand von detaillierteren Modellen mithilfe von FEM-Simulationen ergänzt und spezifiziert. Darin konnte gezeigt werden, dass für den gepulsten Betrieb sowohl die Pulshöhe als auch die Zeitkonstante des gemessenen Widerstands- bzw. Temperaturpulses als Messgröße herangezogen werden kann. Beim 3ω-Betrieb konnte zudem eine starke Frequenzabhängigkeit dieser Widerstands- bzw. Temperaturcharakteristik beobachtet werden. Die analytischen und numerischen Studien wurden anschließend durch experimentelle Ergebnisse bestätigt. Im Hinblick auf die zuvor definierten Kriterien stellte sich heraus, dass die Auswertung der Zeitkonstante beim gepulsten Betrieb im Hinblick auf Sensitivität und Auflösungsgrenze der Auswertung der Pulshöhe deutlich unterlegen ist. Beim 3ω-Betrieb können durch die hochintegrierte Messumgebung deutlich höhere Genauigkeiten und Auflösungsgrenzen erzielt werden. Für den MEMS-Sensor wurde bei einer Anregungsfrequenz von 1 Hz mit 55 ppm die bislang geringste in der Literatur berichtete Auflösungsgrenze für einen thermischen Wasserstoffsensor erreicht. Der Leistungsverbrauch ist dabei im Vergleich zum Pulsbetrieb entsprechend des jeweiligen Tastgrads leicht bis mäßig erhöht, die Ansprechzeit wird bei beiden Operationsmodi maßgeblich durch die Sensorgeometrie beeinflusst. Die geometrischen Parameter der jeweiligen Sensoren haben ferner einen starken Einfluss auf die Sensitivität, womit die Divergenz in der
Performance von MEMS-Chip und Drahtsensor erklärt werden kann. Es besteht ferner eine starke Temperatur- und Feuchtigkeitsabhängigkeit des Sensorsignals, welches bei beiden Operationsmodi indirekt proportional zur Wärmeleitfähigkeit des betrachteten Gasgemischs ist. Die Abhängigkeit vom definierten Umgebungsdruck ist lediglich für die
Zeitkonstante relevant, bei der Pulshöhe besteht keine Druckabhängigkeit. Es konnte eine grundsätzliche Eignung des thermischen Messprinzips für den Einsatz eines Wasserstoffsensors im Automobilbereich festgestellt werden. Für eine Anwendung müssen allerdings Referenzsensoren für Temperatur, Feuchtigkeit und Druck implementiert werden und eine Kalibrierung in Abhängigkeit dieser Größen erfolgen. Der 3ωBetrieb ist aufgrund der höheren Genauigkeit zu bevorzugen. Während der MEMSSensor im Hinblick auf Sensitivität, Ansprechzeit und Leistungsaufnahme gut abschneidet, liegen die Vorteile des Mikrodrahtsensors in seiner Robustheit und der vergleichsweise einfachen Realisierbarkeit
Accurate detection of hydrogen gas in vehicle interiors is very important for the future of a fuel cell car. Since this type of gas is highly volatile and flammable, the measurement methods have to be very reliable and precise due to safety reasons. In this paper a thermal conductivity sensor for hydrogen gas detection is presented, exhibiting a lower detection limit of 2000 ppm hydrogen in laboratory air. The sensor element is realized by micro-fabrication techniques on silicon wafers. The heated filament is exposed by a selective wet etching process creating a micro-hotplate on a thin membrane. In order to minimize power consumption, the sensor is operated in pulsed mode. Hydrogen gas detection was carried out using a synthetic gas testbench. Measurements of hydrogen contents ranging from 0% to 4% with an increment of 0.5% were successfully performed for ambient gas temperatures between -15°C and 84°C. Including humidity, high moisture contents have the greatest influence on thermal conductivity. This was predicted in theoretical investigations and confirmed in experiments. For evaluation, both the change in resistance ΔR as well as the time constant τ were taken as sensor output. For both quantities, the previously established theoretical relationship with thermal conductivity could be confirmed.
Gas concentration monitoring is essential in industrial or life science areas in order to address safety-relevant or process-related questions. Many of the sensors used in this context are based on the principle of thermal conductivity. The 3ω-method is a very accurate method to determine the thermal properties of materials. It has its origin in the thermal characterization of thin solid films. To date, there have been very few scientific investigations using this method to determine the thermal properties of gases and to apply it to gas measurement technology. In this article, we use two exemplary gases (H2 and CO2) for a systematical investigation of this method in the context of gas analysis. To perform our experiments, we use a robust, reliable sensing element that is already well established in vacuum measurement technology. This helix-shaped thin wire of tungsten exhibits high robustness against chemical and mechanical influences. Our setup features a compact measurement environment, where sensor operation and data acquisition are integrated into a single device. The experimental results show a good agreement with a simplified analytical model and FEM simulations. The sensor exhibits a lower detection limit of 0.62% in the case of CO2, and only 0.062% in case the of H2 at an excitation frequency of 1 Hz. This is one of the lowest values reported in literature for thermal conductivity H2 sensors.
We present the application of plasma actuators as a technology for ice prevention at airfoils. The miniaturized dielectric barrier discharge (DBD) plasma actuators (PA) were fabricated by means of microelectromechanical systems (MEMS). We elucidate how to make the actuator samples scalable and applicable to any desired shape by the use of flexible inorganic zirconia substrates. For this purpose, we applied our developed embedding method to integrate the micro actuators in modern carbon/glass fiber reinforced polymer (CFRP/GFRP) materials. Next, the embedded actuator samples were mounted on a mechanical air profile-like fixture and placed in the icing wind tunnel iCORE. The samples were tested in rime ice conditions at temperatures of -15 to -20° C and air speeds up to 30 m/s. Unlike other groups we used a thin film zirconia substrate as dielectric for the plasma actuator. Due to the low substrate thickness of just 150 µm, an operating voltage of 2 kVRMS is already sufficient enough for a stable plasma formation. The experiments show that the operated actuator was able to prevent the ice formation and first indications of a De-icing function were also found. Hence, we show that it is feasible to realize an anti-icing system with zirconia-based plasma actuators operated at lower voltages compared to conventional ones.
Avoiding ice accumulation on aerodynamic components is of enormous importance to flight safety. Novel approaches utilizing surface dielectric barrier discharges (SDBDs) are expected to be more efficient and effective than conventional solutions for preventing ice accretion on aerodynamic components. In this work, the realization of SDBDs based on thin-film substrates by means of micro-electro-mechanical-systems (MEMS) technology is presented. The anti-icing performance of the MEMS SDBDs is presented and compared to SDBDs manufactured by printed circuit board (PCB) technology. It was observed that the 35 mu m thick electrodes of the PCB SDBDs favor surface icing with an initial accumulation of supercooled water droplets at the electrode impact edges. This effect was not observed for 0.3 mu m thick MEMS-fabricated electrodes indicating a clear advantage for MEMS-technology SDBDs for anti-icing applications. Titanium was identified as the most suitable material for MEMS electrodes. In addition, an optimization of the MEMS-SDBDs with respect to the dielectric materials as well as SDBD design is discussed.
Spatially confined non-equilibrium plasmas at vacuum and atmospheric pressure in the dimensions from a few microns to one millimeter are a promising approach to the generation and maintenance of stable glow discharges. The realization of these micro-discharges or micro-plasmas enable more accurate investigations in the field of micro plasma research. We report on field emission assisted micro plasmas excited by an alternating current with frequencies up to 1 kHz. Due to the field enhancement caused by an array of Si nano-tip structures the ignition voltage of stable plasma discharges can be reduced by 30%.
The gasflow created by a minaturized dielectric barrier discharge (DBD) plasma actuator is measured by a MEMS-based thermal conductivity gas sensor giving an indication of flow velocity and flow direction. The possiblity of several sensors in a small area gives a far better accuracy of local flow phenomena compared to conventional sensors. This is important for a better understanding of plasma- induced flow characteristics.
To optimize the measurement range of thermal conductivity vacuum gauges, an expression for the sensitivity is required that takes into account all geometrical, material-specific, and operating parameters. Therefore, equations of the sensor output signal as a function of the pressure for the constant current and the constant temperature mode have been developed analytically. Based on these equations, the sensitivity of the vacuum gauge and its influencing parameters was investigated and discussed. For comparable conditions, the constant temperature operation shows a significantly higher sensitivity for high pressures, while the constant current operation shows higher sensitivity at low pressures. The sensitivity in both the constant current and the constant temperature mode depends on the ratio of the filament surface area and the parasitic thermal conductance. In addition, for the constant current operation, the sensitivity also depends on the current value and the temperature coefficient of the filament resistor. For the constant temperature operation, the sensitivity additionally depends on the distance of the filament and the reference plane. However, to extend the measurement range of a thermal conductivity vacuum gauge toward low pressures, a reduction of the parasitic thermal conductance is mandatory for both the constant current and the constant temperature mode.